The present invention relates to a system for active regulation of the air/gas ratio of a mixture of air and fuel gas fed to a burner, using at least one differential pressure measuring system.
In many kinds of apparatus and installations in which one or more liquid or gaseous fluids circulate, it is often necessary to be able to measure accurately the flowrate of a working fluid and/or the pressure difference between two different working fluids in order to monitor and/or regulate and/or adjust a process. A differential pressure system is usually employed for this purpose, comprising a differential pressure sensor with two inlets connected to respective pressure ports. In the case of measuring the flowrate of a fluid, the two pressure ports are on respective opposite sides of a diaphragm placed in the pipe in which the fluid flows. In the case of measuring the pressure difference between two different fluids, the two pressure ports are connected to respective pipes in which the two fluids flow. In both cases, the accuracy of the measured flowrate or pressure difference depends on the accuracy of the differential pressure sensor, especially at low flowrates and low differential pressures. For example, in the case of a flowrate measurement, the pressure difference xcex94P and the flowrate Q are related by the following equation:
xcex94P=KQ2xe2x80x83xe2x80x83(1)
in which K is a coefficient whose value depends in particular on the density of the fluid whose flowrate is to be measured and on the section of the orifice in the diaphragm placed in the pipe in which said fluid flows. If the instantaneous flowrate of a fluid is to be varied over a wide range, for example in a ratio of 1 to 10, the flowrate of the fluid varies in that ratio but the pressure varies in a ratio of 1 to 100.
In other words, a small variation in flowrate corresponds to a much smaller variation in pressure. The differential pressure sensor used to measure the flowrate must therefore be very accurate and very stable so that it can provide a reliable output value for low flowrates. Differential pressure sensors of this kind exist, but they are extremely costly and therefore cannot be used in apparatus where the total cost of manufacture must remain relatively low, for example in a system for regulating the air/gas ratio of a burner, for example the burner of a boiler for producing domestic hot water and/or central heating hot water.
Also, there are differential pressure sensors which are relatively inexpensive but which are subject to thermal drift and long-term drift which often exceed a few percent. The output signal of such sensors can therefore not be used directly for accurate measurement of the pressure difference over a wide range, for example in a ratio of 1 to 100. If an inexpensive sensor is used, it is therefore often necessary to set the zero of the sensor regularly in order to eliminate the drift referred to above. A conventional solution to this problem uses a measuring system like that shown in FIG. 1 of the accompanying drawings (see also xe2x80x9cPatent Abstracts of Japanxe2x80x9d, Volume 009084, date of publication of the abstract Apr. 13, 1985, and Japanese Patent Application JP59212622 in the name of MATSUSHITA DENKI SANGYO, published Dec. 1, 1984).
The differential pressure measuring system shown in FIG. 1 essentially comprises a differential pressure sensor 1 whose inlet orifices 2 and 3 are respectively connected to a pressure port 4 at which in operation there is a pressure P1 and to the common channel 5 of a 3-channel valve 6. The other two channels 7 and 8 of the valve 6 are respectively connected to a pressure port 9 at which in operation there is a pressure P2 (P2xe2x89xa6P1) and to the inlet orifice 2 of the sensor 1 via a pipe 11. In operation the sensor 1 provides at its output 12 a signal which is representative of the pressure difference P1xe2x88x92P2. That signal is fed to the input of switching means 13, one output 14 of which is connected to a first memory 15 and another output 16 of which is connected to a second memory 17. Although two memories 15 and 17 are shown here, the two memories could be separate memory locations of a single memory. The outputs 18 and 19 of the memories 15 and 17 are respectively connected to the positive and negative inputs of algebraic subtractor or adder means 21 which deliver at their output 22 a measurement signal whose value corresponds to the difference between the output signal values from the sensor 1 respectively stored in the memories 15 and 17.
The valve 6 normally connects the inlet orifice 3 of the sensor 1 to the pressure port 9 and the switching means 13 normally connects the output 12 of the sensor 1 to the input of the memory 15. Under these conditions, the memory 15 stores the value of the output signal of the sensor 1, which corresponds to the difference between the pressures P1 and P2. If the pressures P1 and P2 are equal, the value of the output signal of the sensor 1 should normally be zero. However, as indicated above, inexpensive differential pressure sensors are often subject to thermal drift and long-term drift. Because of such drift the value of the output signal of the sensor 1 is not always zero when the pressures P1 and P2 applied to the two inlet orifices 2 and 3 are equal. Consequently, if the two pressures are different, the value of the output signal of the sensor 1 is subject to an error. That error can be corrected in the following manner. At regular intervals, for example every minute, a control unit 23 sends briefly to the valve 6 and to the switching means 13, via respective lines 24 and 25, control signals which momentarily switch the valve 6 to a state such that it disconnects the inlet orifice 3 of the sensor 1 and the pressure port 9 and connects the inlet orifices 2 and 3 of the sensor 1 and momentarily switch the switching means 13 to a state in which they connect the output 12 of the sensor 1 to the input of the memory 17. Under these conditions, the same pressure P1 is applied to the two inlet orifices 2 and 3 of the sensor 1 and any measurement error of the sensor 1 is stored in the memory 17. The subtractor means 21 subtract that error from the value of the output signal of the sensor 1 stored in the memory 15. Thus the measurement error of the sensor 1 is periodically updated in the memory 17 and a corrected measurement signal is obtained at the output 22 of the subtractor means 21 whose value corresponds to the exact value of the difference between the pressures P1 and P2. The components 13, 15, 17 and 22 therefore form a measurement circuit 26 which, in combination with the 3-channel valve 6 and the control unit 23, enables automatic setting of the zero of the sensor 1.
The prior art differential pressure measurement system described with reference to FIG. 1 is entirely satisfactory from the point of view of setting the zero of the sensor. However, it has the drawback of using a 3-channel valve, which is a relatively costly component.
Differential pressure measuring systems of the type described above can be used in systems for regulating the air/gas ratio of a boiler burner. Systems for regulating the air/gas ratio are described in the Japanese Publication already cited, for example, and in the report published by the Association Technique de l""industrie du Gaz en France [French Gas Industry Technical Association], on the occasion of the 113th Congress du Gaz [Gas Congress], held in Paris on Sep. 10-13, 1996, xe2x80x9cReceuil des Communicationsxe2x80x9d [xe2x80x9cProceedingsxe2x80x9d], Volume 2, pages 245-251, in the article xe2x80x9cRxc3xa9gulation active du rapport air/gaz d""un brûleurxe2x80x9d [xe2x80x9cActive regulation of the air/gas ratio of a burnerxe2x80x9d] by C. PECHOUX et al. The system for regulating the air/gas ratio described in the aforementioned Japanese Publication uses a single differential pressure sensor which measures the difference between the air pressure Pa upstream of the diaphragm in the compressed air supply pipe and the gas pressure Pg downstream of the gas diaphragm in the gas supply pipe. A 3-channel valve and a measuring circuit similar to those described above with reference to FIG. 1 provide automatic setting of the zero of the differential pressure sensor. The system for regulating the air/gas ratio described in the aforementioned report uses two differential pressure sensors, one to measure the difference between the air pressure Pa and the gas pressure Pg, as in the aforementioned Japanese publication, and the other to measure the air flowrate in the compressed air supply pipe. Although in this latter system for regulating the air/gas ratio there is no provision for automatically setting the zero of each of the two differential pressure sensors, this could easily be carried out by associating a two-way valve and a measuring circuit like those described above with reference to FIG. 1 with each of the two sensors. However, a solution of this kind would be relatively costly in that it requires the use of two 3-channel valves and two measuring circuits, one for each sensor.
Patent abstracts of Japan, Volume 008080, date of publication Apr. 12, 1984, and Japanese Patent Application JP58224226, published Dec. 26, 1983, in the name of MATSUSHITA DENKI SANGYO, disclose a system for regulating the air/gas ratio of a burner which uses a single pressure sensor which has a single inlet orifice and is combined with a 3-channel valve so that the sensor alternately measures the air pressure upstream of the air diaphragm and the gas pressure upstream of the gas diaphragm. The pressure sensor is not used as a differential pressure sensor and no means are provided for automatically setting the zero of the sensor.
An object of the present invention is to provide a system for active regulation of the air/gas ratio of a burner using at least one differential pressure measuring system according to the invention.
The differential pressure measuring system employed in the regulation system according to the invention uses a differential pressure sensor which may be subject to thermal drift and long-term drift and includes a measuring circuit for automatically setting the zero of the sensor, said differential pressure measuring system being less costly than the prior art measuring system described above.
The differential pressure measuring system comprises a differential pressure sensor having first and second inlet orifices respectively connected to first and second pressure ports, and an output which, in service, delivers an output signal representative of a pressure difference between the first and second inlet orifices, and a valve which is connected to the first and second inlet orifices of the sensor and which in a first state isolates the two inlet orifices from each other and in a second state connects them to each other, memory means connected to the output of the sensor to memorize at least two values of the sensor output signal. It also comprises a control unit connected to the valve and to the memory means for switching the valve and commanding the storage of a first value of the output signal of the sensor in the memory means when the valve is in its first state and the storage of a second value of the output signal of the sensor in the memory means when the valve is in its second state. It finally comprises measuring means for automatically setting the zero of the sensor.
In a preferred embodiment of the invention, the measuring means consist of memory circuits forming the memory means and subtractor means for calculating the difference between the first and second values of the output signal of the sensor. The measuring circuit delivers at its output a measurement signal representing the exact value of the difference between the respective pressures applied to the first and second inlet orifices of the sensor.
The pressure measuring system further includes a calibrated throttling orifice which is inserted into one of the first and second pressure ports. The valve is a 2-channel valve, a first channel of which is connected to whichever of the first and second pressure ports contains the calibrated throttling orifice, between that calibrated orifice and the corresponding inlet orifice of the sensor. A second channel is connected to the other of the first and second pressure ports. The calibrated orifice has a significantly smaller flow section than that of said 2-channel valve.
With an arrangement of the above kind for setting the zero of the differential pressure sensor, a calibrated throttling orifice and a simple 2-channel valve are used which are easier to manufacture and less costly than the 3-channel valve used in the prior art differential pressure measuring system.
The main object of the invention is therefore a system for active regulation of the air/gas ratio of a burner, comprising an air/gas mixer upstream of the burner, an air pipe containing a calibrated air diaphragm and connected to a first inlet of said air/gas mixer a gas supply pipe containing a calibrated gas diaphragm and connected to a second inlet of said air/gas mixer, both of said pipes being disposed upstream of said calibrated air diaphragm and said calibrated gas diaphragm, means for varying the flowrate of air and means for varying the flowrate of gas sent to said air/gas mixer, and at least one differential pressure measuring system connected to deliver a measurement signal representative of at least one of the following three parameters: the air flowrate in the air pipe, the difference between the air and gas pressures in the air pipe and the gas pipe, and the gas flowrate in the gas pipe, so that the quantity of gas sent to the air/gas mixer is such that the air/gas ratio has a predefined value, wherein each of said differential pressure measuring systems comprises:
a differential pressure sensor having first and second inlet orifices respectively connected to first and second pressure ports, one of which comprises a calibrated throttling orifice, and an outlet which, in service, delivers a signal representative of a pressure difference between the first and second inlet orifices of said sensor,
a 2-channel valve, a first channel of which is connected to whichever of the first and second pressure ports contains said calibrated throttling orifice, between that calibrated orifice and the corresponding inlet orifice of the sensor, and whose second channel is connected to the other of the first and second pressure ports, said calibrated orifice having a flow section significantly smaller than that of said 2-channel valve and said 2-channel valve isolating one of the two inlet orifices from the other when it is in a first state and connecting them to each other when it is in a second state,
memory means connected to the output of each sensor to store at least two values of the output signal of each sensor,
a control unit connected to said 2-channel valve and to the memory means to switch said 2-channel valve and control storage of a first value of the output signal of the sensor in said memory means when the 2-channel valve is in its first state and storage of a second value of the output signal of the sensor in said memory means when the 2-channel valve is in its second state, and
means for calculating the difference between said first and second values of the output signal of the sensor, said memory means and said difference calculating means forming a measurement circuit which delivers at its output a measurement signal representative of the exact value of the difference between the respective pressures at the first and second inlet orifices of each sensor.
In a first embodiment of the system for regulating the air/gas ratio, it is possible to use two differential pressure measuring systems according to the invention to measure the flowrate of air in the air pipe and the difference between the air and gas pressures in the air pipe and in the gas pipe, respectively, each of which two systems includes a differential pressure sensor, a calibrated throttling orifice, a 2-channel valve and a measuring circuit. In this case, two 2-channel valves are used which are simpler and less costly than the two 3-channel valves it is necessary to use with the prior art differential pressure measuring systems.
In another embodiment of the system according to the invention for regulating the air/gas ratio, it is possible to use two differential pressure measuring systems according to the invention to measure the air flowrate and the difference between the air and gas pressures, which two systems share a single calibrated throttling orifice and a single 2-channel valve for setting the zero of each of the two differential pressure sensors.
In a preferred embodiment of the system according to the invention for regulating the air/gas ratio, it is possible to use a single differential pressure measuring system according to the invention to measure the air flowrate and the difference between the air and gas pressures or the gas flowrate, subject to the use of an additional 2-channel valve and switching means for directing the output signal from the measuring circuit of the differential pressure measuring system selectively to the unit for regulating the air flowrate and the unit for regulating the gas supply, the latter regulating unit being designed either in the form of an air/gas pressure regulating unit if the differential pressure sensor of the differential pressure measuring system is designed to measure the difference between the air and gas pressures or in the form of a gas flowrate regulation unit if said differential pressure sensor is designed to measure the gas flowrate.